Tunnel diodes wherein the height of the reduced cross section of the mesa is minimized and process of making



Oct. 29, 1968 KESEL 3,408,275

TUNNEL DIODES WHEREIN THE HEIGHT OF THE REDUCED CROSS SECTION OF THEMESA IS MINIMIZED AND PROCESS OF MAKING Filed Dec. 9, 1966 2Sheets-Sheet 1 Fig.3

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Oct. 29, 1968 c. KESEL 3,408,275

TUNNEL DIODES WHEREIN THE HEIGHT OF THE REDUCED CROSS SECTION OF THEMESA IS MINIMIZED AND PROCESS OF MAKING Filed Dec. 9,. 1.966 2Sheets-Sheet 2 1 y r 55 250 .400 U0. w)

United States Patent 0 3,408,275 TUNNEL DIODES WHEREIN THE HEIGHT (3FTHE REDUCED CROSS SECTION OF THE MESA IS INEMIZED AND PROCESS OF MAKTNGGiinther Kesel, Munich, Germany, assignor to Siemens Aktiengesellschaft,Beriin, Germany, a corporation of Germany Continuation-impart ofapplication Ser. No. 238,898, Nov. 20, 1962. This application Dec. 9,1966, Ser. No. 607,119

11 Ciairns. (Cl. 204-443) ABSTRACT OF THE DISCLQSURE A new process forelectrolytically etching a semi-com ductor component having a p-njunction is shown. In a first step a portion of the p-n junction iscoarsely electrolytically etched with a counter electrode immersed inthe electrolyte. In a line etching step, voltage is applied to oppositesides of the p-n junction without the coaction of the counter electrode.The etching voltage varies between Zero and a value no greater than themaximum operational load of the junction.

The instant application is a continuation-in-part of my abandonedcopending application Ser. No. 238,898 filed Nov. 20, 1962 which isbased upon German application S 76788 of November 22, 1961.

My invention relates to an electrolytic etching method for accuratelyshaping the surface of electronic semiconductor members, and isdescribed herein with reference to the accompanying drawings in which:

FIG. 1 is an explanatory and schematic illustration of a knownelectroyltic method for etching semiconductor members;

FIGS. 2 and 3 are schematic illustrations of mesatype semiconductormembers at the termination of an etching process performed by the knownmethod;

FIGS. 4 and 5 are part-sectional views of mesa semiconductor membersetched according to the invention;

FIG. 6 is an electric circuit diagram of equipment applicable forperforming the method of the invention; and

FIG. 7 is an explanatory current-voltage graph.

The semiconductor members in FIGS. 2 to 5 are shown on greatly enlargedscale.

In the production of semiconductor devices, etching processes areemployed for various purposes, such as for cleaning the surface at thepn junction from any ohmic (conducting) shunts, or for cleaningsemiconductor surface areas to be subsequently contacted with electrodeor terminal metal. Particularly important is the cleaning of the narrowarea where the p-n junction reaches the crystal surface because thesatisfactory and reliable performance of p-n junction devices may dependupon the absence of conductive contamination on that area.

It is an object of my invention to provide an electrolytic etchingmethod capable or reliably performing such cleaning operations,particularly in cases where the semiconductor crystal is of extremelysmall size; and it is another object of my invention to afford usingsuch method also for the purpose of imparting to the semiconductorsurface, particularly to the above-mentioned junction area, the ultimateshape desired at this locality.

The significance of the latter'object will be understood from thefollowing. Semiconductor components for use at high frequencies areoften required to have extremely small dimensions and extremely abruptp-n junctions to meet matching and stability conditions. Notableexamples of such semi-conductor components are tunnel diodes.

Fatented Bot. 29, 1968 ICC The negative and capacitive resistance valuesof the area junction in such devices must permit matching the tunneldiode to the impedances of the external electric circuit. Due to thevery abrupt junctions, tunnel diodes have an extremely high junctioncapacitance, for example, of about 1;tf./cm. so that, when thesemiconductor body of such a tunnel diode is contacted by an alloypellet in the usual manner, a p-n junction capacitance in the order of1000 to 10,000 pf. will result. These high capacitance values could bereduced by giving the alloy pellets smaller dimensions so that thedesired alloying area is attained from the outset. A sufiicientreduction in capacitance, however, would require extremely smalldiameters of the alloy pellets, for example of a few microns, and suchminute pellets are extremely difficult to handle from manufacturingviewpoints.

Another possibility of reducing capacitance is to greatly reduce thejunction area by subsequent treatment. This is done by firstalloy-bonding the metal pellet to the semiconductor crystal andthereafter giving the crystal at the junction area its ultimate shape byelectroiytic etching.

A known method of doing this will be described presently with referenceto FIG. 1. The semiconductor member shown is a tunnel diode comprising agermanium crystal 1 in the form of a plate and a pellet 5 consisting ofindium or a gallium-containing indium alloy. After the pellet 5 isbonded by alloying to the crystal 1, the surface area adjacent to thepellet is partly etched away. The etching effect can be augmented byapplying direct voltage between the semiconductor-pellet member and anelectrode 2 which, like the semiconductor memher, is immersed in theetching solution 3. The positive pole of a direct-voltage source 4,having a voltage of a few volts, is connected to the germaniumcrystal 1. The negative pole is connected to the electrode 2. Under suchcondition, the germanium is anodically oxidized, and germanium becomesdissolved in the liquid. With a suitable etching agent, the alloy-bondedpellet 5, consisting predominantly of indium and being provided with aterminal wire 6, remains unchanged with the exception of a slightsuperficial elimination of material. The etching operation takes placepredominantly beneath the alloy pellet 5 and thus has an undercuttingaction.

With increasing elimination of geranium, the semiconductor memberassumes a shape as typified in FIG. 2. Shown by a broken line is theoutline of the semiconductor crystal 1 prior to etching. Due to etching,the material is removed mainly beneath the pellet 5. It will berecognized that the method produces a mesa on whose flat top the bondedalloy pellet 5 is located. The etching operation just described ishereinafter referred to as rough or coarse etching.

Such coarse etching method involves serious shortcomings which manifestthemselves when the crystal is reduced to very small diameters a of themesa top. The attack of the etching agent takes place uniformly on theentire exposed germanium surface facing the electrode 2 (FIG. 1) so thatthe mesa hill, protected only at its top by the alloy pellet, iscontinuously reduced in cross section by etching along its slopes 7.Before the desired small diameters immediately beneath the alloy pelletare attained, the mesa, whose height is approximately indicated by h inFIG. 2 assumes a strongly tapering shape in comparison with its topdiameter (I. As a result, the mesa has very small mechanical strengthand may break off when subjected to slight disturbances such as totensile stresses at the connecting wire 6. The upwardly tapering shapeof the mesa, required for attaining small current values of the p-njunction, is unfavorable not only for the mechanical reasons mentionedbut also entails electric disadvantages. This is because the taperingportion also constitutes a current supply lead to the p-n junction, buton account of its small thickness this supply lead has considerableelectric resistance which is undesirably connected in series with theinternal resistance and thus causes an additional attenuating effect ofthe semiconductor device. The height h of the mesa is essentially alsodetermined by the size of the alloy-bonded pellet. Large pellets resultin high mesas. Since the height of the mesa determines the current-pathresistance and hence the quality of the etched semiconductor member, themethod necessarily requires the use of alloy pellets of very smalldiameter D if small capacitance values are to be obtained. As mentioned,such small pellet diameters are undesirable because they increase thedifficulties of handling during manufacture.

As is apparent from FIG. 3, the uniform etching effect is particularlydisadvantageous in the vicinity of any wetting gaps 8 at the pellet 5.Such gaps are localities in which there remain hollow spaces due toincomplete wetting of pellet material and of the germanium surface whenthe pellet 5 was alloyed into the silicon material of the semiconductorbody. Such gaps may become exposed by the progressing etching effect andthen result in further mechanical instability such as manifested by theirregular shape of the resulting mesa exemplified in FIG. 3.

The coarse etching method, therefore, encounters technological limitseven before attainment of the desired small capacitance values of 5 pf.and less at maximal current values of 1 to 5 ma. through the junction.

It is therefore a more specific object of my invention to provide anetching method that affords a more reliable production of the desiredultimate shape conjointly with favorable mechanical and electricalproperties of the semiconductor member at its p-n junction, whileavoiding the above-mentioned deficiencies of the known method. An objectalso is to make such improved method applicable with a preceding coarseetching.

To this end, and in accordance with a feature of my invention, thevoltage for a fine electrolytic etching operation is applied directlybetwen the two electric connections or terminals of the p-n junctionand, during progressing electrolytic etching, the current-voltagecharacteristic of the junction is kept under observation, and theetching is continued until this characteristic is adjusted to thedesired, predetermined course.

In contrast to the known method, the electrode denoted by 2 in FIG. 1(or FIG. 6) need not be employed in the fine electrolytic etchingbecause the voltage used therefor is placed as close as possible to thep-n junction by applying it between the semiconductor body 1 and thealloy pellet 5. When thus performing the electrolytic etching, thereoccurs an etched circular groove beneath the alloy pellet as shown at 9in FIG. 4, due to the fact that semi-conductor material from thejunction zone itself has gone into solution.

It is preferable to perform fine etching method according to theinvention, subsequent to a preceding and preferably electrolytic coarseetching operation in which a rather blunt and relatively thick mesa hillis initially produced. Thereafter, the fine etching has the effect ofreducing the upper end 10 of the mesa beneath the alloy pellet (FIG. 5)over its entire periphery without essentially changing the lower slopeOr flank portions of the mesa. As is shown in FIG. 5, the etching attackbeneath the alloy pellet 5 during fine etching is such that, after ashort starting interval, the area beneath the pellet will likewisedecrease approximately in proportion to time, but this decrease takesplace only along a much smaller portion of the mesa slope directlybeneath the pellet 5 than is the case during the preceding coarseetching. The height of the mesa is to a great extent independent of thelateral extent of the alloy pellet because the electrolytic field thatpromotes the etching effect need not act downwardly around the pelletand the undercutting effect of the etching operation depends only uponthe geometric conditions in the immediate vicinity of the junction. Theheight H of the pellet-adjacent mesa portion etched away by fine etchingis slight, being in the order of magnitude of 10 microns, so that theresulting mesa hills have low currentpath resistances. It is to be notedthat the p-n junction, schematically indicated at 11 in FIG. 5, islocated in the area of the step-shaped reduction in mesa diameterresulting from the fine etching effect.

Field distortions due to the fact that potential is applied to theconnecting cable 8 of the alloy pellet cannot occur because the etchingfield is active only in the immediate vicinity of the junction. Theshape and location of the mesa is determined only by the wetting areabetween the germanium crystal and the pellet which defines the positionof the p-n junction. If a hollow space is located beneath the pellet,resulting from insufficient wetting during the alloying process, thenthe fine etching method according to the invention nevertheless resultsin a useful and satisfactory semiconductor member because suchlocalities are not affected by the etching process. The mesa hill of theheight H is produced at a place which is completely wetted because noetching occurs at localities where the junction is interrupted.

The electrolytic fine etching according to the invention can beperformed with the aid of a continuous direct voltage. However, it ispreferable to continuously vary during the entire etching operation theelectrolytic voltage between Zero and the maximum operating voltagelimit of the junction or up to a smaller finite voltage value. For thispurpose a periodically variable voltage, for example an alternatingvoltage, or voltage pulses are preferably impressed between theelectrode pellet and the crystal.

The current flowing during fine etching operation is essentially onlythe electric current passing through the junction. During the continuingetching operation this current through the junction is preferablyindicated by a measuring instrument, as a function of the voltageimpressed across the junction, and the voltage is disconnected tothereby discontinue the etching effect as soon as the instrument showsthat a desired and predetermined current-voltage characteristic isattained. Used for this purpose is preferably an instrument of therecording type that directly records a graph of the current-voltagecharacteristic of the junction in form of a curve. When operating withalternating voltage, a diode is preferably connected in series with thesemiconductor member being subjected to electrolytic etching, in orderto prevent polarity reversal of the etching voltage at the junction. Thejunction and the diode have coincident forward directions of conductanceduring only one-half wave of the alternating etching voltage.

As mentioned, the semiconductor member is subjected to coarse etchingbefore applying the electrolytic fine etching according to the inventionproper. The initial coarse etching may be entirely chemical butpreferably is performed electrolytically with the aid of the sameprocessing equipment subsequently employed for fine etching. Afterperforming the coarse electrolytic etching with the aid of an auxiliaryelectrode as described above with reference to FIG. 1, the ultimate fineetching of the semiconductor member can then be performed withoutchanging the etching bath, simply by switching one pole of the etchingvoltage from the electrode to the electrode pellet.

The processing equipment illustrated in FIG. 6 embodies theabove-mentioned equipment features of the invention. Employed as asource of etching voltage is a transformer 14 whose primary winding isto be connected to a suitable power supply, for example of or 220 v. anda frequency of 50 or 60 c.p.s., and whose secondary furnishes the lowetching voltage desired. Connected to the secondary winding in serieswith a diode 15 and in series with a switch 16 are the semiconductorcrystal 1 and the electrode pellet 5 alloy-bonded to the crystal. Theelectrolyte vessel is provided with an electrode 2 for initial coarseetching when switch 16 is placed into the position indicated by a brokenline. During etching, particularly after termination of the coarseetching stage when the fine etching process is being performed, thecurrent flowing through the pellet-crystal junction is continuouslymeasured by a current-responsive unit A of an instrument 17, and thevoltage impressed across the junction is simultaneously measured by avoltage-responsive unit V of the same instrument. While the two units Aand V may consist of an ammeter and a voltmeter respectively as shownschematically for simplicity, the instrument 17 is preferably a recorderwhose curve-producing means are controlled by a current coil and avoltage coil corresponding to units A and V respectively.

The etching agent used for the method is preferably such that itpossesses only slight or negligible etching action Without applying theelectrolytic voltage. Suitable in this manner is sodium lye of 0.1 toconcentration.

It is preferable to operate with an etching voltage sufficiently high toobtain intensive charge-carrier injection in the junction of thesemiconductor member. This increases the speed of the etching processwhile nevertheless maintaining the etching action limited to thejunction area, particularly to the recrystallization zone.

For the purpose of fine etching according to the invention it is oftenunnecessary to completely immerse the semiconductor member into theetching bath. Another way of proceeding is to deposit upon thesemiconductor surface at the junction, immediately after producing thejunction, an only slight quantity of etching medium, for example bydripping or spraying of the medium, so that only the junction areaitself and the directly adjacent areas of respectively diiferentconductance and/or different conductance type are wetted and covered.For example, a glass tube drawn out to a fine opening can be used fordepositing as many droplets of etching agent upon the junction,previously connected to the voltage source, as are needed to cover onlythe electrode pellet and the immediately adjacent surface portion of thecrystal. The drop of etching liquid covering the junction area becomesmore and more saturated with dissolved semiconductor substance duringelectrolytic etching and this gradually reduces its effectiveness downto a negligible action. Hence the method can be carried out in such amanner that the amount of etching medium applied is just sufiicient tohave the etching medium saturated by disolved semiconductor substance atthe desired termination of the etching operation so that the etchingmedium becomes inactive.

Another mode of performing the method is to similarly control theetching operation by correspondingly selecting the concentration of theetching agent. That is, the concentration is to be so chosen that it issaturated with dissolved semiconductor substance and hence loses itsetching effect at the termination of the process.

An advantage of this saturation process is the fact that the desireddegree of etching and consequently the ultimate shape of the junctionare brought about more and more slowly as the etching process approachesits termination, the etching progress being almost asymptotical with acorresponding choice of the operating conditions, so that the desiredproperties of the junction can be given exact magnitudes and can beuniformly reproduced, particularly with respect to electricalparameters, thus permitting the manufacture of semiconductor members anddevices of uniformly predetermined qualities.

A further mode of performing the fine etching step for ultimatelyshaping the junction and imparting to it the desired electricalproperties is to repeatedly apply etching liquids to the junction withor without simultaneous interruption of the etching current, thuscausing a successive etching away of semiconductor material and acorresponding graduated approach to predetermined electrical qualityvalues. When thus proceeding, the quantity of concentration of theetching liquid placed upon the junction for each individual etching stepis preferably so dimensioned that it virtually loses its etching forcedue to dissolution of semiconductor material before another quantity ofetching liquid is placed upon the junction.

The method according to the invention is of particular advantage in themanufacture of tunnel diodes made in accordance with the alloyingmethod. The electrolytic fine etching permits imparting to such tunneldiodes a desired ultimate shape while also adjusting or modifying theelectric properties, particularly the barrier-layer capacitance, in thedesired manner.

The fine etching method of the invention is further of particularadvantage for accurately adjusting the ultimate value of barrier-layercapacitance and reactive-impedance in varicap diodes and varactordiodes. For this purpose, the etching is performed while observing notonly the current-voltage characteristic of the diodes but alsocontinuously measuring and indicating the barrier-layer capacitance. Thefine etching operation can then be immediately discontinued when apredetermined capacitance value is attained.

I have found it particularly favorable to use an xy oscillograph asindicating instrument (17, FIG. 6) during the fine etching process. Atypical current-voltage curve as shown on the oscillograph isillustrated in FIG. 7, the abscissa indicating voltage in millivolts andthe ordinate indicating current in milliamps.

The etching voltage U during fine etching according to the invention ispreferably increased to a value at which the current I flowing throughthe junction is 10 to times that of I Under such conditions, the rangeof intensive carrier injection, desired for tunnel diodes, is reached.

Described in the following is an example of fine etchng applied afterconventional coarse etching to a tunnel diode having a crystalline bodyof germanium with an indiumalloy pellet as described in the foregoing.The fine etching was performed with sodium lye of 0.1% concentration.Employed for etching was an alternating voltage of 50 c.p.s. and anamplitude of approximately 400 mv. Four drops of etching medium wereapplied in intervals of time onto the surface at the junction asdescribed above. At the beginning of the etching operation caused by thefirst drop, the current I was 58.5 ma. After twenty-four minutes thisvalue had declined to 35.5 ma., at which time the first drop of lye Wasexhausted. The second drop was added, and I,,,.,,,, decreased further by13 ma. but at a slower rate of 0.72 ma. per minutes compare with 0.97ma. per minute during the first stage. A further reduction in rate ofcurrent decline took place during the two subsequent etching steps,namely down to 0.55 and 0.24 ma. per minute respectively. After a totaletching period of 93 minutes, a reduction of I by 55 ma. was obtained.The size of the p-n junction produced by the method was approximately 3microns. The electrical data of the finished tunnel diode were found tohave the following values: valley voltage 250 mv., valley current 0.6ma., current maximum 1.4 ma., voltage at current maximum 55 mv. Themaximum permissible load was about 3 ma.

I claim:

1. The method of electrolytically etching a semiconductor structuralcomponent having a p-n junction at an alloyed-in electrode, wherein theetching process is controlled by the electrical properties of thesemiconductor device, which comprises etching the semiconductor devicein two stages,

(a) coarsely electrolytically etching a portion of the p-n junctionusing a counter electrode immersed in the electrolyte; and

(b) subsequently finely electrolytically etching other portions of thep-n junction by applying the voltage effecting the removal to oppositesides of the pn junction without the coaction of the counter electrode.

2. In the method of claim 1 for electrolytically etching a p-n junctionof semiconductor structural component, wherein the final geometric shapeof the junction and the region immediately adjacent thereto is producedwith a concurrent adjustment of electrical properties of the junction,such as the desired blocking layer capacity, the fine etching step iscarried out by applying a voltage for electrolytic etching at electrodesconnected to opposite sides of the junction, and repeatedly applying theetching means in such an amount and concentration that the fine etchingeffect ends through the dissolving of semiconductor material after ashort time, again applying etching means in the same manner, whileconstantly varying, during the entire fine etching process, the etchingvoltage between zero and a value no greater than the maximum operationalvoltage value of the junction, whereby the junction is reducedconcurrently with the reduction of the peak current carrying capacity toa predetermined value.

3. The process of claim 2, wherein an alternating voltage is applied.

4. The process of claim 2, wherein a periodically changeable voltage isapplied.

5. The process of claim 2, wherein the current through the junction isrecorded as a function of the voltage applied thereto for the fineelectrolytic etching, and interrupting the voltage and thereby theelectrolytic etching upon obtainment of the desired current-voltagecharacteristics.

6. The process of claim 2, wherein voltage for fine electrolytic etchingis increased to the point where the junc- 8 tion is controlled up intothe region of strong carrier injection.

7. The process of claim 5, wherein only a small quantity of etchant isapplied to the junction and the area immediately surrounding thejunction.

8. The process of claim 7, wherein the fine etching consists of a 1-10%aqueous solution of sodium hydroxide.

9. The method of claim 8, wherein the height of the greatly reducedcross section of the mesa does not exceed 10,114.

10. Tunnel diodes produced by alloying, whose final form and electricalcharacteristics are adjusted by electrolytic etching according to claim8.

11. Varicap and varactor diodes, whose blocking layer capacity and theresistance of the p-n junction are adjusted to desired values byelectrolytic etching according to claim 8.

References Cited UNITED STATES PATENTS 2,656,496 10/1953 Sparks 148-153,078,219 2/1963 Chang 204-143 3,081,418 3/1963 Manintveld et al.204-143 3,117,067 1/1964 Jacobs 204-143 3,196,094 7/ 1965 Davis 204-1433,228,862 1/1966 Vulcan 204-143 3,250,693 5/ 1966 Amaya 204-143 ROBERTK. MIHALEK, Primary Examiner.

